Niobium alkoxides, frequently also referred to in the literature as niobium alcoholates, can be utilized for the deposition of corresponding metal oxide layers by means of chemical vapor deposition (CVD) and are therefore valuable starting compounds for producing extremely robust components which find use, for example, in the electronics industry. Such metal oxide layers can also be produced from the corresponding niobium alkoxides via hydrolysis using a sol-gel method. The very high dielectric constant enables, for example, the use of niobium oxide layers in DRAMs (Dynamic Random Access Read/Write Memories). Also, in the lamp industry, niobium oxide which can be deposited by CVD methodologies plays a role, for example, for the production of incandescent bulbs.
However, an existing problem for the electronics industry and also for lamp manufacturers relates to the extreme requirements as to the purity of the starting materials for such layers, i.e., the alkoxides.
The most commonly used, technically simple and economically viable preparation of the niobium alkoxides generally proceeds from the corresponding metal(V) chlorides and alcohols. A comprehensive overview is given by the work “Alkoxo and Aryloxo Derivatives of Metals” by D. C. Bradley, R. C. Mehrotra, I. P. Rothwell and A. Singh, Academic Press, 2001, One such procedure is, for example, described in German Patent Publication No. DE 10 113 169 A1.
Despite some significant chemical similarity to the analogous but virtually colorless tantalum alkoxide compounds, niobium alkoxides (especially niobium ethoxide and higher niobium alkoxides), in contrast, can be obtained only with extreme difficulty in a lighter-colored and hence purer form than the highly colored “crude” niobium alkoxide starting materials. In general, even known distilled niobium alkoxide materials are yellow(-orange) to red-brown in color. For example, a Hazen color number of >280 is a typical measurement for commercially sold niobium ethoxide. This is the case even after repeated distillation. In particular, a reliable, reproducible preparation of light-colored or virtually colorless niobium ethoxide has to date not been possible by distillation alone (vacuum distillation; b.p. 153° C./0.1 mmHg or 200° C./5.5 mmHg).
For example, D. C. Bradley, B. N. Chakravarti, W. Wardlaw; J. CHEM. SOC. 1956, pp. 2381-2384, refer to niobium ethoxide, niobium n-propoxide and niobium n-butoxide as yellow liquids. Using the example of niobium ethoxide, it is recognized that the yellow color cannot be eliminated via repeated distillations. As a route to a higher-purity, only slightly yellow niobium ethoxide, the following complicated process is described: yellow niobium ethoxide is converted in boiling isopropanol to a crystalline mixture of Nb(OEt)(OiPr)4 and Nb(OEt)2(OiPr)3, which is recrystallized four times. Thereafter, this mixed ethoxide-isopropoxide is treated with ethanol four times. Only the distillation of the niobium ethoxide obtained in this complicated manner is described as giving rise to a higher-purity, only slightly yellow product.
Such a complicated process is unusable for an industrial preparation of high-purity, almost colorless niobium ethoxide or higher niobium alkoxides.